This invention relates to system and methods for ion implantation of dopant materials into semiconductor wafers and other workpieces and, more particularly, to systems and methods for implanting workpieces with a rotating ion beam.
Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1000 Angstroms and may eventually require junction depths on the order of 200 Angstroms or less.
The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. However, ion implanters are typically designed for efficient operation at relatively high implant energies, for example in the range of 50 keV to 400 keV, and may not function efficiently at the energies required for a shallow junction implantation. At low implant energies, such as energies of 2 keV and lower, the ion current delivered to the wafer is much lower than desired and in some cases may be near zero. As a result, extremely long implant times may be required to achieve a specified dose, and throughput is adversely affected. Such reduction in throughput increases fabrication costs and is unacceptable to semiconductor device manufacturers.
Another trend in ion implanters is toward single wafer implanters, wherein one wafer at a time is implanted. Batch implanters have been utilized to achieve high throughput, but are large and expensive and place multiple, highly expensive wafers at risk.
One approach to single wafer ion implantation employs a so-called ribbon ion beam. The ribbon ion beam has a width that is at least as great as the diameter of the wafer, and the wafer is mechanically moved in a direction perpendicular to the long dimension of the ribbon beam cross section to distribute the ions over the wafer surface. This approach provides highly satisfactory performance but suffers from certain drawbacks. In particular, the ribbon beam is required to be highly uniform across its width. Compensation components and adjustment procedures are required to provide a uniform beam over a range of beam parameters.
Another well-known approach to single wafer ion implantation employs one-dimensional scanning of the ion beam and mechanical movement of the wafer in a direction perpendicular to the beam scan direction. Yet another approach to single wafer ion implantation employs two-dimensional scanning of the ion beam over the wafer surface. These approaches provide highly satisfactory performance but have drawbacks, including reduced levels of performance at ultralow implant energies.
Accordingly, there is a need for new and improved methods and apparatus for ion implantation of workpieces, such as semiconductor wafers.
According to a first aspect of the invention, apparatus is provided for ion implantation of a workpiece. The apparatus comprises an ion beam generator for generating an ion beam, a deflection device for deflecting the ion beam to produce a deflected ion beam, and a drive device for rotating the deflection device about an axis of rotation to thereby cause the deflected ion beam to rotate about the axis of rotation and to produce a rotating ion beam.
In some embodiments, the deflection device comprises a magnet having polepieces disposed on opposite sides of the ion beam and a magnet winding. A power source supplies a controllable current to the magnet winding. In other embodiments, the deflection device comprises an electrostatic deflection device.
The apparatus may further comprise a controller coupled to the deflection device for controlling deflection of the ion beam. The controller may include means for varying the deflection of the ion beam to thereby cause the rotating ion beam to be distributed over the surface of the workpiece. The controller may comprise means for moving the rotating ion beam in a pattern of concentric annular rings. In other embodiments, the controller includes means for moving the rotating ion beam in a spiral pattern. The controller may further comprise means coupled to the drive device for controlling the rotation of the deflected ion beam. The rotation of the deflected ion beam may be controlled to produce a desired distribution of the ion beam over the surface of the workpiece.
The apparatus may further comprise an angle compensation device for causing the rotating ion beam to have a substantially constant angle of incidence on the workpiece as the deflected ion beam is rotated about the axis of rotation. In some embodiments, the angle compensation device comprises a magnetic lens for directing the rotating ion beam along parallel trajectories. In other embodiments, the angle compensation device comprises an angle corrector magnet and a drive mechanism for rotating the angle corrector magnet about the axis of rotation in synchronism with the rotating ion beam. In further embodiments, the angle compensation device comprises a mechanism for tilting the workpiece in synchronism with the rotating ion beam. In further embodiments, the angle compensation device comprises an electrostatic lens for directing the rotating ion beam along parallel trajectories.
According to another aspect of the invention, a method is provided for ion implantation of a workpiece. The method comprises generating an ion beam, deflecting the ion beam to produce a deflected ion beam, and rotating the deflected ion beam about an axis of rotation to produce a rotating ion beam.